from the GNU version of the Collaborative International Dictionary of English

adj. Dug out of the earth

adj. preserved from a previous geological age; from deep wells; -- usually implying that the object so described has had its substance modified by long residence in the ground, but also used (as with fossil water) in cases where chemical composition is not altered.

adj. Like or pertaining to fossils; contained in rocks, whether petrified or not.

n. A substance dug from the earth.

n. The remains of an animal or plant found in stratified rocks. Most fossils belong to extinct species, but many of the later ones belong to species still living.

n. A person whose views and opinions are extremely antiquated; one whose sympathies are with a former time rather than with the present.

from The Century Dictionary and Cyclopedia

Dug out of the earth: as, fossil coal; fossil salt.

Pertaining to or resembling fossils; preserved by natural inhumation, as an organic body, in form and sometimes in texture: as, fossil shells, bones, or wood. See II., 2.

n. Any rock or mineral, or any mineral substance, whether of an organic or of an inorganic nature, dug out of the ground.

n. Specifically, in later geological and mineralogical use, anything which has been buried beneath the surface of the earth by natural causes or geological agencies, and which bears in its form or chemical composition the evidence that it is of organic origin.

n. Hence, figuratively, one who or something which is antiquated, or has fallen behind the progress of ideas; a person or thing of superannuated or discarded character or quality: as, a curious literary fossil.

Our oxygen does derive from CO2, however, with the remaining carbon mostly spread about in little bits of graphite in the crust, plus a few smidges in higher concentrations, which we call fossil fuels.

A newly unearthed fossil is the missing link between land and marine mammals: Standing two to three feet tall on legs adapted to wade through shallow water, the 48-million-year-old Indohyus is the missing link between modern-day whales and their land-lubbing ancestors.

If we want to hold temperatures below a 2°C rise, the key factor is not how much we burn in fossil fuels each year, but the cumulative emissions over centuries (because once we release carbon molecules from being buried under the ground, they tend to stay in the carbon cycle for centuries).